The present invention relates to a pigment dispersion liquid and a method of producing the pigment dispersion liquid obtained. More particularly, this invention relates to the method of producing a pigment dispersion liquid, which can efficiently produce a pigment dispersion liquid suitable for ink-jet-recording inks with a micro average particle size of 200 nm or smaller.
Inks used for conventional ink-jet-recording systems have mainly been various water-soluble dyes dissolved in water only or in a solvent consisting of water and water-soluble solvents, with various additives if necessary, as disclosed in Japanese Laid Open Patent Publication No. 63-51485, 63-56575, and 01-198671, etc.
However, when printing is done using these dye inks, there is a problem that the dye in the recorded region gets blotted easily if water is spilt on the region, because the recorded image (i.e. the dye ink) on the material on which the image is to be recorded, has low water resistance. Further, there is a problem that change in color tone and decrease in density occur if the recorded region is exposed to light, because lightfastness of the recorded image (i.e. the dye ink) on the material is low.
To solve the above problems of the dye ink, application of so-called pigment inks on ink-jet-recording systems using carbon black or various organic pigments as colorants, instead of using dyes as colorants, has been disclosed in Japanese Laid Open Patent Publication No. 57-10660, 57-10661, 02-255875, 02-276876 and Japanese Examined Patent Publication No. 01-15542.
When printing is done using the pigment ink, since the colorant of the dried ink on the material is a pigment, in contrast to the dye ink, the pigment ink is not dissolved and does not get blotted even when water is spilt on it, having a good water resistance. Moreover, because pigments are less reactive to light in contrast to dyes, pigment inks are more lightfast than dye inks.
The pigment ink is generally prepared as a pigment dispersion liquid by dispersing a mixture of a pigment, a liquid medium, and a dispersant using a dispersing apparatus such as a ball-mill and a sand-mill, and adding various additives if necessary to the prepared pigment dispersion liquid.
Conventionally, in the production of the pigment dispersion liquid, a pigment powder (which normally consists of secondary particles that are aggregates of pigment primary particles), a liquid medium (such as water or organic solvents), and a dispersant are mixed, and the mixture is dispersed with a dispersing apparatus. However, there have been no particular attempts to increase the dispersion efficiency of the mixture.
The pigment particles in the pigment dispersion liquid for ink-jet-recording inks have to be miniaturized to a particle size of 200 nm or less normally and more preferably to a size of 150 nm or less to avoid clogging at the nozzle unfailingly and ensure transparency of the recorded image. To achieve this miniaturized, the pigment powder has had to be miniaturized without limit to a size of the pigment primary particles.
On the other hand, it has been know that the dispersion process of pigment powder consists of steps which are:
(1) xe2x80x9cwettingxe2x80x9d of the pigment powder, that is, a step wherein air on surface of the pigment secondary particles and in voids inside the secondary particles is substituted by the liquid medium and/or the liquid medium plus the dispersant;
(2) a step wherein the pigment secondary particles are ground (or miniaturized) by a dispersing apparatus; and
(3) a step wherein the dispersant is adsorbed on the miniaturized pigment particle surface as a xe2x80x9cprevention of re-aggregationxe2x80x9d of the micro pigment particles.
By performing the xe2x80x9cwettingxe2x80x9d step before performing the dispersion process of the mixture consisting of the pigment powder, the liquid material, and the dispersant, the following dispersion process proceeds efficiently shortening the time required to prepare the pigment dispersion liquid with the micro particle size. However, in the xe2x80x9cwettingxe2x80x9d step in the conventional art, the mixture is only simply stirred for a long period of time (as a step known as premix), and although the xe2x80x9cwettingxe2x80x9d of the pigment secondary particle surface is achieved, in reality the xe2x80x9cwettingxe2x80x9d of the primary particle surface inside the secondary particles cannot be achieved just by simply increasing the stirring time, thus not being able to shorten the time of dispersion.
It is an object of the present invention to provide a method of producing a pigment dispersion liquid, which enables efficient preparation of a pigment dispersion liquid. Another object of the present invention is to provide a pigment dispersion liquid obtained by the method. In particular, it is an object of the present invention to provide the method of producing the pigment dispersion liquid, which can efficiently produce, in a short period of time, a pigment dispersion liquid having a micro particle size suitable for ink-jet-recording inks.
According to one aspect of the present invention, the pigment dispersion liquid is produced using at least a pigment, liquid medium, and a dispersant, as follows. That is, a mixture consisting of at least the pigment, liquid medium and the dispersant is held under a pressure which is less than the atmospheric pressure and then the mixture is held under a pressure equal to or higher than the atmospheric pressure. Finally, a dispersion process is performed.
According to another aspect of the present invention, the pigment dispersion liquid is produced using at least a pigment, liquid medium, and a dispersant, as follows. That is, a mixture consisting of at least the pigment and the liquid medium is held under a pressure which is less than the atmospheric pressure and then the mixture is held under a pressure equal to or higher than the atmospheric pressure. A dispersant is added to this mixture while and/or after the mixture is held under the pressure equal to or higher than the atmospheric pressure. Finally, a dispersion process is performed.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.